Two substrains of Schistosoma mansoni have been derived by selection from our NIH St. Lucian strain, one less and one more infective than the parent strain. Preliminary tests indicate a recently isolated strain of S. mansoni of Puerto Rican origin differs genetically from our original NIH Puerto Rican strain, the new strain having a more restricted range of infectivity for genetically characterized stocks of Biomphalaria glabrata. Crossing experiments have shown that a pathological condition in B. glabrata, with edema of the body resulting in trumpet-like flaring of the shell aperture, is inherited as a simple recessive characteristics. Selection and crossing have shown that amebocytic accumulations observed in certain B. glabrata stocks are genetically determined and that there is an association between these accumulations and insusceptibility to infection with S. mansoni.